Resource management and interference mitigation techniques for relay-based wireless networks

ABSTRACT

Embodiments of a system and methods for resource management and interference mitigation techniques for relay-based networks are generally described herein. Other embodiments may be described and claimed.

REFERENCE TO RELATED INVENTIONS

The present non-provisional application claims priority to U.S.Non-Provisional patent application Ser. No. 12/049,207 filed Mar. 14,2008, entitled “Resource Management and Interference MitigationTechniques for Relay-Based Wireless Networks.”

FIELD OF THE INVENTION

This application relates to relay-based wireless cellular systems and,more particularly, to resource management and mitigation of co-channelinterference in a relay-assisted wireless network.

BACKGROUND

The performance of wireless cellular systems is significantly limiteddue to co-channel interference from neighboring base stations,especially as these systems move towards aggressive frequency reusescenarios. While the overall spectral efficiency of the cellular systemmay improve with aggressive frequency reuse, the performance ofcell-edge users degrades substantially. A variety of interferencemanagement techniques are applied to enhance performance of cell-edgeusers, ranging from the design of fractional frequency reuse (FFR)mechanisms for cell-edge users, to coordinated transmit beam-formingtechniques, to receiver interference cancellation using multipleantennas.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not as alimitation in the figures of the accompanying drawings, in which:

FIG. 1 is a diagram of a relay-based fractional frequency reuse cell,according to some embodiments;

FIG. 2 is a diagram of a cell with cooperative fractional frequencyreuse implemented in the cell;

FIG. 3 is a hierarchical scheduling scheme to enable cooperativefractional frequency reuse, according to some embodiments;

FIG. 4 is a diagram illustrating a relay-based wireless networkenvironment with communication nodes, relay nodes, cooperator nodes, andtarget nodes;

FIG. 5 is a block diagram of an interference mitigation system,according to some embodiments;

FIG. 6 is a diagram of a wireless neighborhood using the interferencemitigation system of FIG. 2, according to some embodiments;

FIG. 7 is a flow diagram showing the co-channel interference avoidanceof the interference mitigation system of FIG. 2, according to someembodiments;

FIG. 8 is a flow diagram of the transmission randomization of theinterference mitigation system of FIG. 2, according to some embodiments,and;

FIG. 9 is a flowchart of a method for providing resource management andinterference mitigation in a relay-based wireless network environment.

DETAILED DESCRIPTION

Embodiments of methods and systems for providing interference mitigationand resource management in a relay-based wireless cellular network aredescribed herein. In the following description, numerous specificdetails are set forth such as a description of a combined use ofcooperative relay communications, relay-based fractional frequencyreuse, and relay-based probabilistic interference mitigation to providea thorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that the invention can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of the invention.

A limitation to performance in wireless networks is poor reliability andcoverage caused by random fluctuations due to fading in wirelesschannels. Cooperative downlink communication techniques in a relay-basedcellular wireless network allow multiple relay stations and possibly abase station to jointly transmit information to multiple users, allowingthe extraction of multiple-input multiple-output (MIMO) benefits in adistributed fashion, including gains of cooperative diversity,cooperative multiplexing and distributed array (i.e., power efficiency).This makes cooperative relay communication an ideal technique forthroughput, coverage and reliability enhancement in cellular wirelessnetworks.

It would be an advance in the art to increase throughput, capacity, andcoverage improvements in a wireless network, particularly for userslocated at or near boundary regions of sectors, or cell edges, formed asa result of base transceiver station and/or relay station antennaconfiguration and orientation and consequently suffering poorperformance due to low signal-to-interference-and-noise ratioconditions. In the downlink mode, performance improvements for thesecell-edge users may be enabled by using cooperative transmissiontechniques whereby multiple relay stations and possibly the base stationinteract jointly to share their antennas to extract MIMO benefits in theform of cooperative diversity, cooperative multiplexing and distributedarray gains. Alternatively, fractional frequency reuse may be employedin both downlink and uplink modes, to provide wireless networkimprovements by reducing cell-edge interference caused by repeated useof a given frequency in a number overlapping communication channels.Further, wireless network improvements may be provided by randomizingtransmissions to cell-edge users by carefully controlling theprobability of transmission to the cell-edge users as a way to reduceinterference in a wireless network. A relay-based fractional frequencyreuse policy that can support cooperative transmissions usingprobabilistic interference mitigation techniques among multipleinfrastructure terminals, such as base transceiver stations and relaystations, may provide for improved wireless network performance. Relayand access links may be separated in frequency as well as in time.Consequently, users in a wireless network may enjoy the combinedbenefits of interference mitigation, cooperative diversity, and powerefficiency.

Now turning to the figures, a diagram of a relay-based fractionalfrequency reuse cell 100 according to some embodiments is described inFIG. 1. In one embodiment, the relay-based fractional frequency reusecell 100 with a serving base station 105 is surrounded by six relaystations 110, 112, 114, 116, 118 and 119. In another embodiment, threerelay stations may be used, though generally, any number of relaystations may be placed in the relay-based fractional frequency reusecell 100 at arbitrary locations. The relay stations transmit and receivesignals to and from the serving base station 105 and/or to other relaystations and/or to mobile stations to improve the quality ofcommunication to the mobile stations located in areas near the edge ofthe relay-based fractional frequency reuse cell 100. To provide powersavings at the relay stations 110, 112, 114, 116, 118 and 119, andminimize co-channel interference, one consideration of the relay stationdeployment is such that relay station coverage areas do not overlap.

Depending on a location in the cell 100, a given mobile station (MS)will be associated to the serving base station 105 or one or more of therelay stations 110, 112, 114, 116, 118 and 119. For example, a MS at thecenter of cell 100 is likely to be directly connected to the servingbase station 105 while a MS at an edge of a cell 100 is likely to beconnected to one or more relay stations 110, 112, 114, 116, 118 and 119.Further, depending on the MS location, different spectrum reuse policiesmay be adopted over the radio access links, wherein the radio accesslink may be between the serving base station 105 and MS or between arelay station 110, 112, 114, 116, 118 and 119 and MS, including (i)spectrum reuse by the serving base station 105 and one of the relaystations 110, 112, 114, 116, 118 and 119, (ii) spectrum reuse bymultiple relay stations 110, 112, 114, 116, 118 and 119, and (iii)spectrum reuse by the serving base station 105 and multiple relaystations 110, 112, 114, 116, 118 and 119.

Multi-hop relaying and spectrum reuse techniques allow for throughput,capacity and coverage improvement, but resulting interference due tosimultaneous transmissions of the serving base station 105 and relaystations 110, 112, 114, 116, 118 and 119 over radio access links shouldbe managed to avoid performance losses due to severe interferenceissues. Mobile stations at cell 100 edges may suffer from interferenceproblems. Similarly, users located at edges of the coverage areas ofrelay stations 110, 112, 114, 116, 118 and 119 may also suffer fromsimilar interference problems, not only caused by co-channelinterference from other cells (i.e. all stations outside cell 100), butalso intra-cell interference caused by the MSs inside cell 100, i.e.,serving base station 105 and/or the relay stations 110, 112, 114, 116,118 and 119. To address this problem, a reuse factor for a given MSwithin the cell 100 should not only be adjusted with respect to itsgeographical location with respect to the serving base station 105, butalso its location with respect to the relay stations 110, 112, 114, 116,118 and 119.

In FIG. 1, the frequency reuse and channel allocation schemes over therelay-based cell 100 are allocated according to frequency channelregions 120, 130, 140, 150, 160 and 170. At the center of the fractionalfrequency reuse cell 100, the serving base station 105 operates in afirst frequency channel 120 region surrounded by a second frequencychannel region 130. In this embodiment, the purpose of using a secondfrequency channel region 130 is to lower frequency reuse in comparisonto the first frequency channel region 120 and thereby reduce co-channelinterference seen by mobile stations in this region, so that the mobilestations can receive better quality of service in terms of throughput,capacity and coverage. Such kind of frequency reuse at the center of thefractional frequency reuse cell 100 provides improved communications toMSs located near the serving base station 105 by assigning resourcesamong two channels with varying degrees of reuse in the center of therelay-based fractional frequency reuse cell 100.

Relay stations 110 and 116 operate near the cell 100 edge in a thirdfrequency channel 140 region surrounded by a fourth frequency channel150 region to provide enhanced communications to MSs that may otherwiseexperience heavy co-channel interference. Relay stations 112 and 118operating in the third frequency channel 140 region surrounded by afifth frequency channel 170 region and relay stations 114 and 119operating in the third frequency channel 140 region surrounded by asixth frequency channel 160 are similarly configured in this embodimentto provide fractional frequency reuse (FFR) within the cell 100. Relaystations 110, 112, 114, 116, 118 and 119 may use the same frequencychannel for the mobile stations located proximate to each relay stationto maximize spectral efficiency benefits from frequency reuse. Further,each relay station is also able to use different frequency channels formobile stations that are at the edges of their respective coverageareas. Thus, the mobile stations in the frequency channel regions 150,160 and 170 are served with lower frequency reuse in comparison to thosemobile stations in the frequency channel region 140. Lowering frequencyreuse in these regions of cell 100 reduces co-channel interference seenby mobile stations and thereby enhances throughput, capacity andcoverage. On the other hand, full fractional reuse may be applied to thecenter channel regions corresponding to the third frequency channel 140immediately adjacent to the relay stations in addition to FFR applied tochannel regions located around the perimeter of the center channelregions.

In some embodiments, one or more of the frequency channel regions 150,160 and 170 may correspond to the same frequency channel, implying moreaggressive frequency reuse among relay stations 110, 112, 114, 116, 118and 119 for users at the edges of their coverage areas. Moreover, insome embodiments, frequency channel regions 120 and 140 may correspondto the same frequency channel, implying more aggressive frequency reuseamong serving base station 105 and relay stations 110, 112, 114, 116,118 and 119 for the users served under high frequency reuse. Finally, insome embodiments, one or more of the frequency channel regions 130, 150,160 and 170 may correspond to the same frequency channel, implying moreaggressive frequency reuse among base station 105 and relay stations110, 112, 114, 116, 118 and 119 for the users served under lowerfrequency reuse.

In some embodiments, a MS situated at a cell 100 edge would be servedunder frequency reuse of three with three orthogonal bands (or sets ofsub-channels) allocated to pairs of relay stations each located onopposite sides of the cell 100 to avoid co-channel interference, i.e.,as depicted in FIG. 1, the frequency channel regions 150, 160 and 170would then correspond to three separate frequency bands. In an alternateembodiment, a reuse pattern of two may be applied to provide twosub-channels to each of three relay stations in an alternating pattern.In a further embodiment, a reuse pattern of six may be used to provideeach relay station with its own frequency channel, requiring a total ofsix orthogonal channel allocations.

In the embodiment shown in FIG. 1, MSs located very close to the servingbase station 105 (within the first frequency channel 120) are servedunder a reuse 1 policy and the cell 100 edge users located very close toone of the relay stations 110, 112, 114, 116, 118 and 119 (within thethird frequency channel 140) are served under a reuse 1 policy.Alternately, a reuse 3 policy is adopted for MSs which are far from theserving base station 105 (within the second frequency channel region130) and any relay stations 110 and 116 (within the fourth frequencychannel 150 region), 112 and 118 (within the fifth frequency channel 170region), and 114 and 119 (within the sixth frequency channel 160) toensure that a cost of resulting interference from spectrum reuse isminimized for these MSs.

FIG. 2 is a diagram of relay-based fractional frequency reuse cell 100illustrated in FIG. 1 with cooperative fractional frequency reuse,resulting in a cooperative FFR cell 200. Downlink (DL) cooperativerelaying techniques are combined with relay-based FFR concepts with theobjective of enabling successful interference management in acooperative relay mode to benefit MSs in a cellular system located atthe cell edge or periphery that normally experience poor SINRconditions. Resultant cooperative diversity and power efficiency gainscan lead to a significant performance gain for cell-edge MSs; which isroughly comparable to that provided by receive maximal ratio combining(MRC) techniques.

In the presence of a relay-based FFR policy as shown in FIG. 2, theinclusion of DL cooperative relay support for cell edge MSs implies thatMSs in the reuse 3 zones (i.e. within the second frequency channelregion 130, the fourth frequency channel 150 region, the sixth frequencychannel 160, or the fifth frequency channel 170 region) may be served bymultiple infrastructure terminals (serving base station 105 and/or relaystations 110, 112, 114, 116, 118 and 119) through their simultaneoustransmissions using cooperative relaying techniques such as distributedspace-time coding (STC), distributed beam-forming, cooperativemultiplexing, etc.

Cooperative relaying techniques are applied to provide a target node 220with a poor signal to interference and noise ratio (SINR) with enhancedinterference management capabilities in FIG. 2. The resultantcooperative diversity and power efficiency gains lead to a significantincrease in performance for the target node 220 (for example a mobilestation in the form of a cellular phone, personal digital assistant(PDA), pocket PC, handheld computer device, etc.). Cooperative diversityresults from multiple infrastructure terminals (relay stations 110, 112,114, 116, 118 and 119, serving base station 105, etc.) providingsimultaneous or nearly simultaneous transmissions using a cooperativerelaying technique such as distributed space-time coding (STC),distributed beam forming, cooperative multiplexing, etc.

A combination of relay-based FFR and DL cooperative diversity enablesthe target node 220 to simultaneously realize the interferencemitigation advantages of FFR and cooperative diversity, along with powerefficiency advantages of cooperative relaying, for cooperative FFR cell200 edge mobile stations is referred to as cooperative FFR (coop-FFR).In one embodiment, the target node 220 operates using cooperativediversity in a downlink (DL) mode in coop-FFR region 210 located inand/or between the fifth frequency channel 170 and sixth frequencychannel 160. As a result, the target node 220 may receive an assignedtime-frequency resource for reception from relay station 118 while aneighboring relay station 119 will be given the option to transmit onthe same time-frequency resource to support DL cooperative relaying.

A difference between coop-FFR and traditional relay-based FFR in thereuse 3 policy mode is that a cooperative FFR cell 200 edge mobilestation may receive simultaneous cooperative transmissions on the sametime and frequency zone from two or more adjacent infrastructureterminals (relay stations 110, 112, 114, 116, 118 and 119, serving basestation 105, etc.) in coop-FFR while this is not allowed usingtraditional FFR. For example, when a MS, such as the target node 220 isassigned to a particular time-frequency resource for reception from aninfrastructure terminal, the neighboring infrastructure terminals willalso be given the option to transmit on the same time-frequency resourceto support DL cooperative relaying. Therefore, the cooperative FFR cell200 edge mobile station not only avoids the dominant interference fromthe adjacent infrastructure terminals, the infrastructure terminals arealso used to advantageously realize cooperative diversity and powerefficiency gains.

Downlink data is sent by the serving base station 105 and all the relaystations 110, 112, 114, 116, 118 and 119, can receive the DL data andlearn the scheduling decisions of the serving base station 105.Furthermore, in the case of orthogonal frequency-division multipleaccess (OFDMA) based resource allocation in which the serving basestation 105 schedules the data intended for different relay stations110, 112, 114, 116, 118 and 119 orthogonally in a relay zone, each relaystation can hear the data and control information (MAPs etc.) of otherrelay stations. Therefore, the overhead cost of cooperation is expectedto be very small in terms of required additional bandwidth to schedulefor cooperative relay transmissions. While there may be some additionaloverhead cost in terms of the control information (e.g. additional MAPsetc.) to schedule and coordinate the DL cooperative relay transmissions,known remedies can be applied to minimize such costs.

FIG. 3 is an embodiment of a hierarchical scheduling and resourcemanagement scheme to enable cooperative fractional frequency reuseinvolving a communications transmit node, two relay nodes and acommunications receive node. An embodiment of a communications transmitnode 410, a first relay node 420, a second relay node 430 and acommunications receive node 440, or target node, and cooperator node 450are illustrated in FIG. 4. The communications transmit node 410 may bethe serving base station 105, the communications receive node 440 may bethe target node 220 the relay nodes, 420 and 430, may be any two of therelay stations 110, 112, 114, 116, 118 and 119, and the cooperator node450 or neighbor node may be a mobile station, a subscriber station 560(FIG. 5), or another base station or relay node. Development ofscheduling and resource management techniques for relay-assistedcellular networks is necessary in order to support DL cooperativerelaying and advanced relay-based interference management policies. In acooperative relaying and the coop-FFR scheme, simultaneous transmissionsof multiple relay stations 110, 112 114, 116, 118 and 119 or of theserving base station 105 and one or more relay stations 110, 112, 114,116, 118 and 119 are required to transmit in a coordinated fashion sothat they occur in the same time/frequency (TF) and with a coordinatedMCS and a chosen cooperative transmission protocol.

While centralized scheduling at the serving base station 105 canaccomplish this task, it is not a preferred approach due tocomplications that arise in ranging, bandwidth request and networkentry. Instead, a two-layer hierarchical (or hybrid) scheduling schememay be used. A first layer is distributed scheduling, which servesmobile stations in a non-cooperative fashion. The second layer iscentralized scheduling, which serves a specific set of MSs (such ascooperator node 450) that benefit from cooperative relaying. While theresource allocation for most MSs relies on distributed scheduling, somelevel of centralized coordination by the base station is possibleregarding the scheduling decisions for the MSs that can benefit from DLcooperative relaying. In this context, the relay stations 110, 112, 114,116, 118 and 119 create their own schedules using distributedscheduling, but the serving base station 105 may create specificallocations for the relay stations 110, 112, 114, 116, 118 and 119downlink transmissions to enable limited centralized coordination andhierarchical scheduling. Each relay station 110, 112, 114, 116, 118 and119 is notified about the relevant centralized scheduling decisions ofthe serving base station 105 so that it can perform its distributedscheduling on the time-frequency resources not assigned by the servingbase station 105.

The hybrid scheduling scheme to enable coop-FFR uses a centralizedcoordination mechanism (at the base station) to classify all users to beserved in a relay-assisted fashion into two groups: (i) mobile stationsthat will be served cooperatively, e.g., those mobile stations that areat relay station cell edges under the reuse 3 allocation and areselected by the base station for cooperative transmission; (ii) mobilestations that will be served non-cooperatively, e.g., 1) those usersvery close to a particular relay station so that they are under reuse 1allocation, and 2) those users that are at relay station cell edgesunder the reuse 3 allocation, but the base station decides that thesemobile stations should not be served cooperatively.

This grouping of the users may be performed based on criteria such asuser location information, channel quality indicator (CQI) metrics andtraffic loads at the base station and relay stations, and may bemaintained across multiple frames or changed on a frame-by-frame basis.

The relay stations should also be informed about these decisions so thateach relay station knows: (i) which mobile stations it will servecooperatively and which mobile stations it will serve non-cooperatively,and (ii) which time-frequency (TF) zones have been assigned forcooperative transmissions and which TF zones are to be used fornon-cooperative transmissions.

Over the TF zones allocated for non-cooperative transmissions, eachrelay station may perform distributed scheduling for the set of mobilestations it is instructed to serve non-cooperatively and a base stationdoes not have to help with the specific TF resource assignments forthese mobile stations.

Over the TF zones allocated for cooperative transmissions, furthercentralized coordination by the base station will be necessary tospecify the user TF resource assignments, cooperative transmissionschemes and MCS choices and this information should be conveyed to therespective relay stations to be inserted into their DL-MAPs. Thisapproach limits the use of centralized scheduling only for the mobilestations that should be served cooperatively and the remaining mobilestations can be served via distributed scheduling.

An example for the hierarchical scheduling scheme to enable the coop-FFRscheme is shown in FIG. 3 for a two-hop relay-assisted DL communicationsetting. In this figure, the COOP-MAP zone in DL-MAP carries theinformation about a base station's centralized scheduling decisionsregarding the mobile stations served under cooperative relay protocols.In the next DL subframe, the relay stations include COOP-MAP in theirown DL-MAPs, but also create their own allocation for mobile stationsserved under distributed scheduling.

Returning to the figures in FIG. 3, the communications transmit node 410transmits a data block 310 with a header or preamble 312 and a bodycontaining a fundamental channel (FCH) 314 to provide basic data serviceto data users, a downlink (DL) map 316, a cooperator map (COOP-MAP) 318including scheduling and modulation coding scheme information, an uplinkmap (UL-MAP) 320, relay station 1 data 322, relay station 2 data 324,and cooperator data 326. The preamble 312 contains supplemental data,placed at the beginning of the data block 310, used for framesynchronization and may contain data block 310 handling information. Thefirst relay node 420 of FIG. 4 receives a first relay node data block330 containing a first relay node data block header 332 and first relaynode data packets 334 and 336.

The second relay node 430 of FIG. 4 receives a second relay node datablock 340 containing a first relay node data block header 342 and firstrelay node data packets 344 and 346. The data block 310 is transmittedand the first relay node data block 330 and the second relay node datablock 340 are received in a first DL sub-frame 308.

In a second DL sub-frame 348, the first relay node 420 transmits to thecommunications receive node 440 (target node or subscriber station), afirst relay node data block 350 with a first relay node preamble 352,first relay node fundamental channel 354, first relay node DL MAP 356,first relay node COOP-MAP 358, first relay node UL-MAP 360, first relaynode COOP-DATA 362, and relay station 1 data 364. Similarly, in thesecond DL sub-frame 348, the second relay node 430 transmitscommunications receive node 440, a second relay node data block 370 witha second relay node preamble 372, second relay node fundamental channel374, second relay node DL MAP 376, second relay node COOP-MAP 378,second relay node UL-MAP 380, second relay node COOP-DATA 382, and relaystation 2 data 384. The COOP-MAP 358 and COOP-DATA 362 from the firstrelay node 420 and the COOP-MAP 378 and COOP-DATA 382 from the secondrelay node 430 allow a communications receive node 440 (as well as othermobile stations that are served cooperatively in the same region) tocommunicate using cooperative fractional frequency reuse. In someembodiments, only one relay station may transmit the whole COOP MAP tothe mobile stations that are served cooperatively in the same region,while both relay stations would transmit COOP-DATA.

The discussed hierarchical scheduling and resource management techniquecould be also be applied to general relay-based cellular networks tosupport transmission techniques other than cooperative relaying, such assuccessive interference cancellation techniques, which also requirescentralized coordination by a base station. As before, distributedscheduling can be performed for the users which will not be served byany advanced cooperative relay or advanced interference managementtechniques.

The communications receive node 440 may communicate using according tocellular-based communications using an appropriate cellular standardsuch as a general packet radio system (GPRS), enhanced data rates forglobal evolution (EDGE), or third-generation wireless (3G), though theembodiment is not so limited. In other embodiments, other wirelesscommunication standards may be employed, such as but not limited tocommunications defined by the Institute of Electrical Institute ofElectrical and Electronic Engineers (IEEE) 802.11, Wireless Fidelity(Wi-Fi) and IEEE 802.16 Worldwide Interoperability for Microwave Access(WiMAX) suites of standards.

In a relay-assisted cellular network, interference management becomesmore complex since there are multiple sources of interference. Forinstance in the DL mode, the interference observed by a mobile stationmay be due to (i) co-channel interference from other base stations, (ii)co-channel interference from the relay stations in neighboring basestation cells and (iii) intra-cell interference from otherinfrastructure terminals (base station and/or relay stations). Theplurality of sources causing interference makes it difficult to mitigateinterference in a coordinated fashion since a huge amount ofcoordination would be needed to simultaneously avoid interference anduse TF resources in an efficient manner. In addition, the design ofinterference mitigation approaches using techniques such as MIMOmulti-user detection (MUD) and/or successive interference cancellation(SIC) techniques is also more difficult since the number of antennadegrees of freedom (in MIMO-MUD) and IC-cancellation layers (in SIC)required to track/mitigate possible interferers is large. Moreover,techniques such as cooperative relaying involve simultaneoustransmissions by multiple infrastructure terminals in a given TFresource, increasing the potential interference to other transmissionsat the same TF. Under such difficulties encountered with interferencemitigation approaches based on coordinated transmissions, a use ofrandomized transmissions and probabilistic interference mitigationtechniques may be used in conjunction with the coop-FFR scheme.

In one example, the MS identifies the base-stations and relay stationscausing the most interference to transmissions on its link to thedesired base-station or relay stations. This information along withother parameters (e.g. channel quality indicators, interferencemeasurements, number of dominant interferers etc.) is reported by themobile station to its relay station or base station and in case of arelay station, the relay station relays the information to the basestation. The base station shares the identity of the base stations andrelay stations causing the most interference, or interference beyond agiven channel threshold, along with other relevant parameters with otherbase stations, which in return may inform their respective relaystations about this information. At the end of this coordination period,each base station and relay station knows the links on which it needs torandomize transmissions and what probabilistic decision making criteriashould be considered for various actions. The set of links to berandomized are controlled by a probabilistic interference mitigationmedium access control (PIM-MAC). The probabilistic decision makingcriteria for various actions may be based on average SINR conditions,determined by system geometry and location of mobile stations, as wellas quality of service (QoS) demands, traffic conditions and userpriorities and therefore require only periodic updates and coordinationamongst base-stations and relay stations. Different probabilisticdecision making criteria may be designed for different actions such asrouting, cooperative relaying and link adaptation.

Each base station and relay station determines actions to be chosen perPIM-MAC link, based on various observed system and channel parametersand pre-determined quality thresholds; using the developed probabilisticdecision making criteria.

Probabilistic interference mitigation in a relay-based cellular networkleads to the following methods: i) Probabilistic relaying and routing: Arelay station that causes too much interference to a neighboring basestation cell or relay station cell may not be selected for routing by abase station scheduler, even though the employed routing algorithm mayindicate that the multi-hop route through this particular relay stationyields the best end-to-end link quality.

ii) Probabilistic cooperation and mode selection: A relay station withfavorable channel qualities to assist another relay station for purposesof DL cooperative relay transmission to a mobile station may not bescheduled by the base station with a certain probability since the basestation learns that this relay station causes too much interference to aneighboring base station cell or relay station cell. Similarly, thedetermination of which cooperative relay mode should be used or whethercooperation should not be employed is performed in a random fashion withpre-determined probabilities assigned to each mode. Apart from theselection of the cooperative relay modes, the probabilistic modeselection may also be applied toward (i) relay-based FFR, i.e. fordetermining the reuse factor (e.g. reuse 1 vs. reuse 3) to be used invarious zones of the relay station cells, and (ii) deciding whetheradvanced interference mitigation techniques should be used in variouszones in the base station cell and relay station cells.

In one embodiment, a system for mitigating interference in a wirelesscommunications network comprises a communications node configured tocommunicate with a target node at a first frequency band and time slotthrough a multi-hop route comprising a cooperator node, a relay nodeconfigured to communicate with the target node, the relay node operatingat a second frequency band and time slot, and a medium access controllerconfigured to randomize a plurality of transmission from thecommunications node for co-channel interference avoidance, based atleast on a channel threshold value.

An interference mitigation system 500 is depicted in FIG. 5 according tosome embodiments. The interference mitigation system 500 includes aserving base station 105, a relay station 118 and a subscriber station560. The serving base station 105 and the serving relay station 118 aretypically selected by the subscriber station 560, based on the relativestrengths of the signals received by the subscriber station 560 fromserving base station 105 and/or relay station 118. The interferencemitigation system 500 may include one or more other base stations,denoted as base station 502, base station 504, and base station 506. Theserving base station 105 has a medium access controller (MAC) 532, therelay station 118 has a MAC 582, and the subscriber station 560 has aMAC 562. The MACs 532, 582 and 562 include functional and structuralcomponents not described herein, which are well-known to those ofordinary skill in the art. These functional and structural components,which are common to all base stations in the wireless region, are knownherein as legacy MAC operations.

In some embodiments, the MACs 532, 582 and 562 each include novelcomponents suitable for co-channel interference avoidance (CIA), knownas the CIA MAC 580. Because the MACs 532, 582 and 562 continue tosupport other MAC functions not described herein, all of the servingbase station 105, the relay station 118 and the subscriber station 560have both legacy MAC and CIA MAC 580 functionality.

The CIA MAC 580 includes co-channel interference avoidance 570,transmission randomization 540, transmission randomization 590, physicallayer optimization 550 and physical layer optimization 595, in someembodiments. As shown in FIG. 5, co-channel interference avoidance 570is performed by the subscriber station 560 while transmissionrandomization 590 and physical layer optimization 595 are performed bythe relay station 118 and transmission randomization 540 and physicallayer optimization 550 are performed by the base stations 502, 504, 506and 105.

A wireless neighborhood 600 is depicted in FIG. 6, to facilitateunderstanding the interference mitigations system 500 of FIG. 5,according to some embodiments. The wireless neighborhood 600 includesseven cells 610, each of which has a base station, BS₁-BS₇(collectively, base stations BS). Subscriber stations 560, depicted asmobile devices, denoted M₁, M₂, . . . M₉, are employed throughout thewireless neighborhood 600 (collectively, subscribers M). The number ofsubscribers M may vary over time. Lines c₁ show the desired linksbetween mobile subscribers M and base stations BS. For mobile subscriberM₁, there exists a desired link, c₁, to the base station, BS₁. Becausethe base stations are transmitting on channel 1 (c₁) using the samefrequency, such transmission may cause interference to mobile stationsin other cells. For example, in FIG. 6, interferences on the samechannel are occurring from base stations, BS₄ and BS₂, indicated asi_(c1) in both cases.

The interference mitigation system 500 commences with the subscriberstation 560 (or target node). The subscriber station 560 notifies theserving base station 105 and the relay station 118 of interference fromsome other base station or relay station in the wireless neighborhood600. In FIG. 5, a feedback link 534 is shown pointing from thesubscriber station 560 to the serving base station 105 to indicate thisstep. Then, the serving base station 105 shares the interferencereport(s) with the other base stations in the wireless neighborhood 600that there has been a report of interference. Interference reportinglinks 520 are shown in FIG. 5 between the serving base station 105 andeach of the base stations 502, 504, and 506 in the interferencemitigation system 500. Once all base stations in the wirelessneighborhood 600 are aware of the interference, the base stationsdetermine whether to perform transmission randomization 540, in someembodiments. Like other transmissions, randomized transmissions occuraccording to parameters obtained through physical layer optimization550. Moreover, the relay station 118 determines whether to performtransmission randomization 590, in some embodiments. Like othertransmissions, randomized transmissions occur according to parametersobtained through physical layer optimization 595.

Co-channel interference avoidance 570 operates according to the flowdiagram of FIG. 7, in some embodiments. The operations are performed bythe subscriber station 560 (or one of the subscribers M in the wirelessneighborhood 600), although the operations may be performedsimultaneously by multiple subscribers. The subscriber station 560identifies the base station 502 (or relay stations) causing the mostinterference to transmissions on its link to the serving base-station(block 700). The subscriber station 560 then makes a determinationwhether to notify its serving base station 105 and possibly its relaystation 118 of the interference by comparing the interference-to-carrierratios (ICR) to a threshold (Γ) (block 710).

If the threshold is not exceeded (block 720), the interference is notsufficient to trigger the notification by the subscriber station 560.Otherwise, the subscriber station 560 submits the identity of thebase-station(s) and relay station(s) causing the most interference toits serving base station 105 and possibly its relay station 118 (block730). In some embodiments, the submission operation constitutes one ormore exchanges of the CIA MAC trigger information between the subscriberstation 560, the serving base station 105 and the relay station 118. Asused herein “CIA MAC trigger” means events that lead the subscriberstation 560 or the serving base station 105 or the relay station 118 toinvoke the CIA MAC 580 of their respective MACs 562, 582, 532. In otherwords, the CIA MAC trigger is when the subscriber station 560 determinesthat the interference exceeds the threshold.

The serving base station 105 communicates the information reported bythe subscriber station 560 to other base stations in the wirelessneighborhood 600 (block 740). At this point, each base station knows thelinks in which interference has been reported, which are the links inwhich transmissions may optimally be randomized (block 750). In someembodiments, the CIA MAC trigger may be based on average SINRconditions, determined by system geometry and location of subscribers.In these embodiments, the CIA MAC trigger is updated and coordinatedamong other base stations in the wireless neighborhood periodically.

Once the CIA MAC 580 is triggered, the transmission randomization 540 ofthe MAC 532 in the serving base station 105 and the transmissionrandomization 590 of the MAC 582 in the serving relay station 118 areinitiated. The other base stations and relay stations in the wirelessneighborhood likewise initiate transmission randomization to thesubscriber station 560. FIG. 8 is a flow diagram showing operationsperformed to randomize transmissions in the wireless neighborhood 600,according to some embodiments. The operations in FIG. 8 may be performedby all base stations BS and all relay stations in the wirelessneighborhood 600, but, for simplification, only one base station and onerelay station are indicated in the flow diagram.

The base station and relay station identify the links to be analyzed(block 800), which are the links between the serving base station 105,the relay station 118 and the subscriber station 560 that reported theinterference. The base station and relay station determine channelthresholds for the CIA MAC links (block 810). If the channel gain on agiven link does not exceed a “channel threshold” (block 820), notransmission to the subscriber station 560 occurs (block 830).Otherwise, the serving base station 105 and the serving relay station118 transmit to the subscriber station 560 with optimized physical layerparameters (block 840). Hence, in some embodiments, the transmissionprobability is proportional to the probability of exceeding the “channelthreshold”.

Before the transmission randomizations 540, 590 and physical layeroptimizations 550, 595 can take place, however, the interferencemitigation system 500 determines the threshold for the CIA MAC trigger.In some embodiments, each subscriber station 560 makes a decision totrigger the CIA MAC 580. This trigger is based on comparing the measuredinterference-to-carrier ratio (ICR) from each base-station to athreshold. In some embodiments the threshold is derived based on theassumption of one strong interferer per subscriber. The extension tomultiple interferers is straightforward. In some embodiments, wherethere is a single interferer, the optimal threshold is derived bycomparing the goodput of a system using the CIA MAC 580 with that of asystem having no CIA MAC. For the optimal threshold, the good-put of CIAMAC is greater than the good-put of a traditional MAC. The values of theICR thresholds are calculated as a function of the SNR.

The interference mitigation system 500 may employ alternate methods fortriggering the CIA MAC 580. These include but are not limited tocomparing good-put based on more than one strong interferer, usinglocation-based information or cooperation between subscriber stations560 to determine severely interfered users, etc. Interference mayfurther be avoided through implementation of fractional frequency reuse,described earlier.

FIG. 9 is a flowchart of a method for providing resource management andinterference mitigation in a relay-based wireless network environment.In element 900, a serving base station 105 identifies one or more relaystations, such as relay station 110, and one or more mobile stations orsubscriber stations including a target node 220 in a coverage zoneillustrated in FIG. 1 as cell 100. The identified mobile stations areclassified as cooperative users or as non-cooperative users in element910. The serving base station 105 determines whether each relay stationhas cooperative user mobile stations in element 920. If the serving basestation 105 does not have cooperative user mobile stations, the servingbase station 105 instructs one or more relay stations to performdistributed scheduling for the non-cooperative user mobile stations(element 930). If the serving base station 105 does have cooperativeuser mobile stations, the serving base station 105 informs the one ormore relay stations of the cooperative user mobile stations and anynon-cooperative user mobile stations (element 940). The serving basestation 105 then performs centralized transmission scheduling for thecooperative user mobile stations (element 950) and communicatestransmission scheduling information to the one or more relay stationsand instructs one or more relay stations to perform distributedscheduling for the non-cooperative user mobile stations (element 960).The serving base station 105 receives interference parametric data(element 970) and transmits the interference parametric data to the oneor more relay stations and neighboring base stations (element 980).Serving base station and relay station transmissions with a target node220 are randomized using probabilistic interference mitigation mediumaccess control in element 990.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.Embodiments of the invention propose techniques for resource managementand interference mitigation in relay-based cellular networks. Theproposed algorithms and architectures enable resource managementtechniques such as hierarchical scheduling and advanced interferencemitigation techniques such as fractional frequency reuse (FFR) andprobabilistic interference mitigation to work in conjunction with relayprotocols (e.g., cooperative relaying) and therefore allow realizing theperformance advantages from all of these techniques simultaneously.

While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. In the description andclaims, the terms “coupled” and “connected,” along with theirderivatives, may have been used. It should be understood that theseterms are not intended as synonyms for each other. Rather, in particularembodiments, “connected” may be used to indicate that two or moreelements are in direct physical or electrical contact with each otherwhile “coupled” may further mean that two or more elements are not indirect contact with each other, but yet still co-operate or interactwith each other.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

Thus, embodiments of this invention may be used as or to support asoftware program executed upon some form of processing core (such as aprocessor of a computer) or otherwise implemented or realized upon orwithin a machine-readable medium. A machine-readable medium includes anymechanism for storing information in a form readable by a machine (e.g.,a computer). For example, a machine-readable medium can include such asa read only memory (ROM); a random access memory (RAM); a magnetic diskstorage media; an optical storage media; and a flash memory device, etc.

Modifications may be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification and the drawings. Rather, the scope ofthe invention is to be determined entirely by the following claims,which are to be construed in accordance with established doctrines ofclaim interpretation.

What is claimed is:
 1. A base station configured to communicate with a plurality of mobile stations and a plurality of relay nodes, wherein one or more of the plurality of mobile stations comprise a first group to be served cooperatively using centralized scheduling through a first relay node of the plurality of relay nodes and one or more of the plurality of mobile stations comprise a second group to be served non-cooperatively through a second relay node of the plurality of relay nodes using distributed scheduling, wherein the second group of mobile stations is to be served using a frequency reuse that is greater than one, the base station comprising a scheduler to schedule time-frequency resources for the first group of mobile stations and wherein the base station is configured to notify the one or more relay nodes to perform distributed scheduling for the second group of mobile stations using time-frequency resources not assigned to the first group of mobile stations and wherein the second relay node of the plurality of relay nodes is configured to randomize transmissions to the second group of mobile stations.
 2. The base station of claim 1, wherein the base station is configured to transmit a data block comprising a fundamental channel, a downlink map, and a cooperator map.
 3. The base station of claim 2, wherein the cooperator map comprises scheduling and modulation coding scheme information determined at the base station.
 4. The base station of claim 2, wherein the base station communicates through time division duplexing.
 5. The base station of claim 1, wherein the plurality of transmissions from the base station are centrally scheduled to serve the first group of mobile stations using cooperative relaying through a number of other relay stations.
 6. The base station of claim 1, wherein the base station is configured to communicate with the plurality of mobile stations using a centralized scheduling scheme and a distributed scheduling scheme and wherein the base station is further configured to communicate directly with the plurality of mobile stations.
 7. The base station of claim 1, wherein the base station is configured to serve the first group cooperatively by sending packets to the first group of mobile stations from the base station and the first relay station.
 8. The base station of claim 7, wherein distributed scheduling means that the second relay node schedules resources for the second group of mobile stations. 